This invention relates to an ignition timing control system for an internal combustion engine for controlling the ignition timing of the engine at engine start.
Ideally, a spark ignition-type engine should operate at optimum ignition timing under all running conditions to fully combust the fuel mixture and develop maximum output power at all times. Experiments have shown that the ideal crank angle position at which cylinder internal pressure peaks for maximum engine power is about 12 degrees after top dead center. Accordingly, the conventional practice is to set ignition timing so that the position of the indicated pressure peak assumes the ideal crank angle position. However, the engine will not always develop maximum power because the actual position of the indicated pressure peak is not necessarily brought to the ideal crank angle position every cycle owing to a fluctuating ignition delay between an ignition operation and the actual ignition of the mixture. An attempt to improve upon the foregoing is a method of controlling ignition timing in dependence on a phase difference between an electrically detected position of the peak value of cylinder internal pressure and an electrically detected predetermined angular position of the engine output shaft. Though the method is an effective one, applying it at engine start-up time causes unstable engine rotation and difficulty in achieving a reliable start. The reason is that when the engine is started by a starting motor, a motoring waveform is superposed on the indicated pressure waveform, so that the true position of the indicated pressure peak is difficult to recognize. This can cause the position of a motoring waveform peak to be mistakenly recognized as the position of the indicated pressure peak, with the result that the ignition timing is so controlled as to assume an improper value.